Add template events viewtopic_body_polls

I propose 3 template events:

1:

EVENT viewtopic_body_poll_question_prepend

It would be the equivalent of viewtopic_topic_title_prepend, and will allow us to use prefixes, or better yet, icons. It will allow us to use code like:

<h2><span class="<!-- IF S_CAN_VOTE and L_POLL_LENGTH -->poll-icon-timer<!-- ELSEIF S_CAN_VOTE -->poll-icon-open<!-- ELSE -->poll-icon-closed<!-- ENDIF -->">{L_VIEW_TOPIC_POLL}</span>....</h2>

2:

EVENT viewtopic_body_poll_fieldset_prepend

Having control of the poll fieldset can have advantages in case we want to add extra functionality, buttons or create wrapping containers.

3:

EVENT viewtopic_body_poll_fieldset_prepend

The counter-part to the previous one